Let a b be the side where the primary shadow is, and b c the primary light, d will be the spot where it is intercepted,_f g_ the derived shadow and f e the derived light.

And this must be at the beginning of the explanation.

[Footnote:  In the original MS. the text of No. 252 precedes the one given here.  In the text of No. 215 there is a blank space of about four lines between the lines 2 and 3.  The diagram given on Pl.  VI, No. 2 is placed between lines 4 and 5.  Between lines 5 and 6 there is another space of about three lines and one line left blank between lines 8 and 9.  The reader will find the meaning of the whole passage much clearer if he first reads the final lines 11—­13.  Compare also line 4 of No. 270.]

On relative proportion of light and shadows (216—­221).

216.

That part of the surface of a body on which the images [reflection] from other bodies placed opposite fall at the largest angle will assume their hue most strongly.  In the diagram below, 8 is a larger angle than 4, since its base a n is larger than e n the base of 4.  This diagram below should end at a n 4 8. [4]That portion of the illuminated surface on which a shadow is cast will be brightest which lies contiguous to the cast shadow.  Just as an object which is lighted up by a greater quantity of luminous rays becomes brighter, so one on which a greater quantity of shadow falls, will be darker.

Let 4 be the side of an illuminated surface 4 8, surrounding the cast shadow g e 4.  And this spot 4 will be lighter than 8, because less shadow falls on it than on 8.  Since 4 faces only the shadow i n; and 8 faces and receives the shadow a e as well as i n which makes it twice as dark.  And the same thing happens when you put the atmosphere and the sun in the place of shade and light.

[12] The distribution of shadow, originating in, and limited by, plane surfaces placed near to each other, equal in tone and directly opposite, will be darker at the ends than at the beginning, which will be determined by the incidence of the luminous rays.  You will find the same proportion in the depth of the derived shadows a n as in the nearness of the luminous bodies m b, which cause them; and if the luminous bodies were of equal size you would still farther find the same proportion in the light cast by the luminous circles and their shadows as in the distance of the said luminous bodies.

[Footnote:  The diagram originally placed between lines 3 and 4 is on Pl.  VI, No. 3.  In the diagram given above line 14 of the original, and here printed in the text, the words corpo luminoso [luminous body] are written in the circle m, luminoso in the circle b and ombroso [body in shadow] in the circle o.]

217.

THAT PART OF THE REFLECTION WILL BE BRIGHTEST WHERE THE REFLECTED
RAYS ARE SHORTEST.